Conventionally, RFID systems where a reader/writer transmits a radio wave of approximately 1 W, a tag side receives the signal and returns information within the tag with a radio wave, and the reader/writer identifies the tag, have been put into practical use.
The tag includes an antenna and an LSI chip connected to the antenna. The tag does not include a power supply within the tag itself, but activates the circuit of the LSI chip by using power induced by resonance with communication waves from a reader/writer, and transmits an ID and the latest updated data within a memory to the reader/writer.
In such an RFID system, a wireless signal of the UHF (Ultra High Frequency) band (865 MHz in EU, 915 MHz in US, and 953 MHz in Japan) is used.
A communication distance of an RFID system using such a UHF band is relatively long, and is expected to be utilized in various fields in the future.
However, if the tag is attached to a metal such as a personal computer, an automobile, a container, a steel desk, etc., or an object including a liquid, such as a polyethylene terephthalate bottle, a human body, etc., the antenna gain of the tag is deteriorated by mirror image effects peculiar to the metal being a good conductor of electricity or the liquid, leading to a significant deterioration in the communication distance of the tag. A solution to this problem has been demanded.
In light of the above described background, various types of tag antennas adaptable to metals or liquids have been devised so far, and some of them have been commercialized.
For example, Patent Document 1 as such conventional technology describes a configuration for preventing a patch antenna from being affected by an object existing on the side of a ground conductor by arranging the ground conductor in a position opposed to the patch antenna with respect to an interposed dielectric, and by arranging a tag to make the ground conductor contact the object including a liquid or a metal.
However, to implement the configuration described by Patent Document 1, an LSI chip must be connected to the patch antenna and the ground conductor, which are arranged above and under the interposed dielectric. This connection is made using a method of installing a connection wire to go around the side surface of the dielectric, or using a method of forming a penetration hole in the dielectric and of inserting a connection wire into the penetration hole. Both of these methods require cumbersome process steps.
In the meantime, Patent Document 2 as conventional technology which does not require such cumbersome process steps describes a method by which an LSI chip can be connected to a patch antenna with only a process executed on the surface of a dielectric.
However, for the tag antenna described by the above described Patent Document 1 or 2, an expensive material such as a high-frequency substrate, ceramics, etc. is used as a dielectric onto which the tag antenna is pasted. Therefore, further reductions in product price are desired so as to meet growing demand expected in the future. Moreover, the demands for a further increase in the communication distance and a broader band of available frequencies have been increasing.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-157905 (FIGS. 1-4 and 6-8)    Patent Document 2: U.S. Pat. No. 6,215,402 B1 (FIGS. 3, 4A and 4B)